Secondary batteries such as lithium ion secondary batteries have advantages such as high energy density, small self-discharge, excellent long-term reliability and the like, and therefore they have been put into practical use in notebook-type personal computers and mobile phones. More recently, the development of the high performance secondary battery having further improved capacity and energy density is demanded due to, in addition to the trend of high functionality of electronic equipment, the expansion of market of motor driven vehicles such as electric vehicles and hybrid vehicles and the acceleration of the development of domestic and industrial power storage systems.
However, in the batteries having high capacity and high energy density, the temperature rise of the battery is liable to occur when an external shock is applied or they are in overcharged state. In particular, in a battery containing a positive electrode active material having high capacity and high energy density, for example compounds of layered rock-salt structure containing nickel or cobalt, there is a tendency that energy released during thermal decomposition is increased, and therefore, the consideration to safety is furthermore needed.
To increase the safety of batteries of high energy density, there have been conducted various studies so far. In particular, with respect to the heat generation in batteries, there have been conducted studies on cut off mechanism to stop the function of the battery when the battery temperature rises. For example, Patent Document 1 discloses a lithium battery comprising a positive electrode plate, a negative electrode plate and a heat insulating layer disposed on the charge-discharge surface of the electrode plates, the lithium battery being capable of reducing the electrical conductivity when the temperature of the lithium battery increases. Patent Document 2 discloses a lithium battery having a thermal actuation protective film formed on the material surface of a positive electrode plate or a negative electrode plate, in which if the temperature of the lithium battery rises to the thermal actuation temperature of the thermally actuation protective film, the thermal actuation protective film undergoes a cross-linking reaction to prevent thermal runaway. Patent Document 3 discloses a lithium battery comprising an electrode plate formed of a plurality of the electrode material layer and comprising a thermally activatable material in at least one of these electrode material layers, in which when the temperature of the lithium battery rises, the thermally activatable material is activated to undergo cross-linking reaction, thereby reducing the electrical conductivity. In the batteries disclosed in these Patent Documents 1, 2 and 3, the heat insulating layer (Patent Document 1), the thermal actuation protection film (Patent Document 2) and the electrode material layer (Patent Document 3), respectively, comprises a nitrogen-containing polymer formed by the reaction of bismaleimide monomer and barbituric acid, and the conductivity of the batteries are lowered by such a mechanism that when the temperature of the batteries rises, the polymer is converted into a cross-linked polymer which inhibits the diffusion of lithium ions. In other words, these batteries provide a shutdown function to batteries by using heat-reactivity of maleimide group of the polymer.